The supplementary materials for the article "Four billion people facing severe water scarcity" by Mesfin M. Mekonnen and Arjen Y. Hoekstra provide a detailed discussion on the assumptions and methods used in previous studies to estimate water scarcity. The authors highlight that previous studies often underestimated water scarcity due to evaluating it at too large spatial scales (e.g., river basins) or annual rather than monthly basis, and by excluding environmental flow requirements. They argue that these limitations hide the true extent of water scarcity in specific areas and during specific periods. The study proposes a more accurate measure of water scarcity by estimating it at a high spatial and temporal resolution (30 × 30 arc min) and considering environmental flow requirements and net water consumption. This approach better captures the true fraction of water available for consumptive activities that has been consumed, leading to a more reliable estimate of severe water scarcity.The supplementary materials for the article "Four billion people facing severe water scarcity" by Mesfin M. Mekonnen and Arjen Y. Hoekstra provide a detailed discussion on the assumptions and methods used in previous studies to estimate water scarcity. The authors highlight that previous studies often underestimated water scarcity due to evaluating it at too large spatial scales (e.g., river basins) or annual rather than monthly basis, and by excluding environmental flow requirements. They argue that these limitations hide the true extent of water scarcity in specific areas and during specific periods. The study proposes a more accurate measure of water scarcity by estimating it at a high spatial and temporal resolution (30 × 30 arc min) and considering environmental flow requirements and net water consumption. This approach better captures the true fraction of water available for consumptive activities that has been consumed, leading to a more reliable estimate of severe water scarcity.